Means for reducing the memory effect in a mass spectrometer ion source



' Filed Aug. 17, 1960 MEANS FOR REDuIN THE MEMORY EFFECT'IN 'A- SPECTROMETER ION SOURCE July 28, 1964 A N HAME-R ETAL ,7

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INVENTORS July 28, 1964 A. N. HAMER E TAL 3,142,752

. MEANS FOR REDUCING THE MEMORY EFFECT IN A MASS SPECTROMETER ION SOURCE Filed Aug. 17, 1960 2 Sheets-Sheet 2 FIG. 3

wry ///%/W\\ INVENTOQE BY @MMM United States Patent M 3,142,752 MEANS FOR REDUKIING THE MEMORY EFFECT IN A MASS SPECTROMETER ION SOURCE Allan N. Hamel, Bury, and Colin C. McCormick, Wallasey, England, assignors to United Kingdom Atomic Energy Authority, London, England Filed Aug. 17, 1960, Ser. No. 50,147 Claims priority, application Great Britain Aug. 17 1959 Claims. (Cl. 250-413) This invention relates to mass spectrometers suitable for use with gaseous samples such as uranium hexafluoride.

In the analysis of the isotopic concentration of uranium hexafiuoride using a mass spectrometer an adverse memory effect is observed. In this memory efiect, the measured isotopic composition obtained on one sample is afiected by the presence of deposits from previous samples, the measurements on the sample under analysis being predominantly affected by the immediately preceding sample.

It has now been established that this memory eitect is in part caused by an isotopic exchange. Thus when a sample of uranium hexafiuoride is introduced to the ion source, corrosion of th ion source walls by the uranium hexafiuoride can occur leading to the deposition of uranium tetrafiuoride. When the following sample of uranium hexafiuoride is introduced, isotopic exchange can occur between the uranium hexafluoride and the deposited uranium tetrafiuoride (via intermediate fluorides such as uranium pentafiuoride) thereby giving rise to the presence of traces of uranium hexafluoride having the isotopic composition of the original sample.

It has also been established that the memory effect is partly caused by adsorption of uranium hexafluoride on the wall of the pipe used to introduce the sample to the ion source, the adsorbed uranium hexafluoride being released when the following sample is being passed through the pipe to the ion source.

According to the invention a mass spectrometer suitable for use with gaseous samples such as uranium hexafiuon'de comprises an ion source compartment housing an ion source, and a pipe for introducing a gaseous sample through a small orifice in the ion source to intersect the ionizing beam of the ion source, wherein the, pipe has its sample discharge and in close proximity to but spaced from the ion source and the bore of the pipe is smaller than said orifice so that substantially all the sample passes directly from the pipe through the orifice and the pipe is thermally separated from the ion source so reducing corrosion in the region of the ion source to reduce isotopic exchange memory effects.

Preferably the pipe has its sample discharge end formed of ceramic material.

By way of example, the invention will now be described with reference to the accompanying drawings wherein:

FIG. 1 is a longitudinal view partly in medial section,

FIG. 2 is a view of the ion source of FIG. 1 in the direction of arrow II of FIG. 1, and

FIG. 3 is a section on the line III-III of FIG. 2.

Referring to FIG. 1 a mass spectrometer for use with gaseous uranium hexafiuoride samples comprises an ion source compartment 1 having a sealed end plate 2 and housing a molecular beam ion source 3. A pipe 4 for introducing a uranium hexafiuoride sample through a small orifice 23 in the ion source 3 passes through the plate 2, the pipe 4 being sealed to the plate 2 in passage therethrough and having its sample feed end 5 close to the plate 2. The pipe 4 has its sample discharge end 6 in close proximity to but spaced from the ion source 3.

3,142,752 Patented July 28, 19 64 The pipe 4 comprises a copper pipe 7 having an enlarged end 8 in which a ceramic (e.g., alumina) tube 9 is located as a push fit. Mounted from the plate 2 is an ion source support column 10 shown in outline form. The ion source 3 and its associated accelerator electrode 11 and focusing electrode 12 are supported from the column 10 by dowels 13 (indicated also by center-lines 27), the dowels being fitted with insulating spacers 28 for insulating the ion source 3 and the electrodes 11, 12 from each other and from the column 10.

Referring to FIGS. 2 and 3 in addition to FIG. 1, the ion source 3 comprises a circular base plate 15 to which a copper electrode 16 is secured by bolts 17. The electrode 16 comprises a rectangular base 18 having two integral plates 19 defining a channel 20, each plate 19 having a central aperture 21 and two end apertures 22. The base 18 has the orifice 23 defined therein, the orifice 23 communicating with a circular recess 24 defined in the base plate 15 and the base 18, the orifice 23 and recess 24 being concentric with the bore of the pipe 4 (FIG. 1). The ion source 3 has a filament cathode and an anode to provide an ionizing electron beam in the channel 20, the direction of firing of the electron beam being indicated by dotted arrow 25 (FIG. 1). The plate 15 has holes 14 for the dowels 13, the electrodes 11, 12 having similarly positioned holes.

Dimensions of the mass spectrometer described above with reference to the drawings are as follows:

The copper pipe 7 has an efiective length of 4.9 inches and an inside diameter of A inch.

The ceramic tube 9 is 20 mm. long by 1.7 mm. inside diameter by 2.7 mm. outside diameter.

The end 5 of the pipe 4 is 1.4 inches from the plate 2.

The orifice 23 has a diameter of 2 mm. and a length of The recess 24 has a depth of 3 mm.

The radial clearance between the outer Wall of the tube 9 and the side wall of the recess 24 is 1 mm.

The clearance between the end 6 and the end wall of the recess 24 is 1 mm.

The plate 15 is 10 mm. from the end of the column 10,

and 8.5 mm. from the electrode 11.

The electrode 12 is 2 mm. from the electrode 11, and the electrodes 11, 12 have a thickness of 0.75 mm.

The plate 15 is 0.75 mm. thick and 3.5 cms. in diameter.

The channel 20 is 15 mm. long, 3.5 mm. Wide, and 4 mm. deep.

The mass spectrometer described above has an evacuating system connected to the compartment 1, insulated electrical leads 29 sealed in passage through the plate 2 by seals 30 and passing through the column 10 to the ion source 3 and associated electrodes, and may be used in conjunction with the collector head described in copending British application No. 8555/58 corresponding to US. application Serial No. 798,387, filed March 10, 1959, now issued as US. Patent No. 2,961,538.

In use, a uranium hexafluoride sample passed into the pipe 4 from the end 5 is introduced to the ion source 3 through the orifice 23, the bore of the pipe 4 being smaller than the orifice 23 so that substantially all the sample passes directly from the pipe 4 through the orifice 23. The dimensioning of the pipe 4 promotes molecular flow of the gas which is beamed into the channel 20. In the channel 20 the gas intersects, and is ionized by, the ionizing electron beam, the direction of firing of the resultant ionized beam being indicated by arrow 26 (FIG. 1).

The use of the ceramic tube 9 prevents disturbance of the electrical field in the channel 20 which might other- Wise occur if the tube 9 were of metal construction. The clearance between the end 6 and the walls of the recess 24 is such that any small amount of gas issuing from the end 6 and striking the end wall of the recess 24 instead of passing through the orifice 23 is readily pumped away from the area of the ion source 3 by the evacuating system.

The ion source 3 operates at a temperature of about 200 0, this heating being due to the filament cathode. The spacing of the end 6 ensures that the pipe 4 is thermally separated from the ion source, and since corrosion increases with increasing temperature, thereby reduces corrosion in the region of the ion source to reduce isotopic exchange memory effects.

The positioning of the end 5 close to the plate 2 of the compartment 1 enables the length of the pipe 4 to be reduced to reduce adsorption memory effects.

The annular part of the base 18 between the channel 20 and the recess 24 in the region of the end 6 defines a partition (0.5 mm. thick) serving to define the electrical conditions in the source region by preventing penetration of the electrical field in the channel 20 into the recess 24.

We claim:

1. in a mass spectrometer ion source assembly suitable for use with gaseous samples capable of isotopic exchange, the combination comprising an ion source compartment end plate; an ion source mounted from the end plate, the ion source comprising a base and plates defining a channel for an electron beam, the base having an orifice therein communicating with the channel; and a pipe having a bore concentric with and smaller than the orifice for introducing a gaseous sample to the ion source to intersect the electron beam, the pipe being sealed to the end plate in passage therethrough and having a sample discharge end proximate to but spaced from the base such that substantially all the sample passes from the discharge end through the orifice and into the channel.

2. A mass spectrometer ion source assembly as claimed in claim 1, wherein the sample discharge end of the pipe is formed of ceramic material.

3. In a mass spectrometer ion source assembly suitable for use with gaseous samples capable of isotopic exchange, the combination comprising: an ion source compartment including an end plate; an ion source means housed within the compartment and including a base plate and an electrode base mounted on the base plate, the electrode base including side plates defining a channel on one side thereof for an electron beam, the base plate and electrode base defining a recess within the base plate and the electrode base on the side of the electrode base opposite the channel, and the electrode base defining an orifice joining the recess and the channel; and a pipe having a bore concentric with and smaller than the orifice for introducing a gaseous sample to the ion source to intersect the electron beam, the pipe being sealed to the end plate in passage therethrough and having a sample discharge end within the recess and proximate to but spaced from the base such that substantially all of the sample passes from the discharge end through the orifice and into the channel.

4. The combination according to claim 3 wherein the electrode ba'se includes an annular partition which defines the recess and prevents penetration of any electrical field in the channel into the recess.

5. The combination according to claim 3 wherein the sample discharge end is spaced from the base by a distance of about one millimeter.

References Cited in the file of this patent UNITED STATES PATENTS 2,551,544 Nier et al. May 1, 1951 2,911,531 Rickard et al. Nov. 3, 1959 2,958,775 Robbins et al. Nov. 1, 1960 2,977,495 Klein Mar. 28, 1961 

1. IN A MASS SPECTROMETER ION SOURCE ASSEMBLY SUITABLE FOR USE WITH GASEOUS SAMPLES CAPABLE OF ISOTOPIC EXCHANGE, THE COMBINATION COMPRISING AN ION SOURCE COMPARTMENT END PLATE; AN ION SOURCE MOUNTED FROM THE END PLATE, THE ION SOURCE COMPRISING A BASE AND PLATES DEFINING A CHANNEL FOR AN ELECTRON BEAM, THE BASE HAVING AN ORIFICE THEREIN COMMUNICATING WITHT HE CHANNEL; AND A PIPE HAVING A BORE CONCENTRIC WITH AND SMALLER THAN THE ORIFICE FOR INTRODUCING A GASEOUS SAMPLE TO THE ION SOURCE TO INTERSECT THE ELECTRON BEAM, THE PIPE BEING SEALED TO THE END PLATE IN PASSAGE THERETHROUGH AND HAVING A SAMPLE DISCHARGE END PROXIMATE TO BUT SPACED FROM THE BASE SUCH THAT SUBSTANTIALLY ALL THE SAMPLE PASSES FROM THE DISCHARGE END THROUGH THE ORIFICE AND INTO THE CHANNEL. 